1. Field of the Invention
The present invention relates to a multi-axis magnetic lens and variants thereof used for focusing a plurality of charged particle beams individually and in parallel. More particularly, it relates to a multi-axis magnetic lens acting as an objective lens or a condenser lens or a transfer lens in a multi-beam apparatus which uses a plurality of electron beams to in parallel expose patterns onto or inspect defects on a wafer or mask in semiconductor manufacturing industry.
2. Description of the Prior Art
In semiconductor manufacturing industry, using an electron beam to expose patterns onto or inspect defects on a wafer or a mask has been a powerful method when critical feature dimensions of patterns or defects are beyond the competent ability of a photon beam. The reason is that an electron beam can offer superior spatial resolution compared to a photon beam due to its short wavelength. However, such a superior spatial resolution will be fundamentally deteriorated by electron interaction or called as Coulomb Effect as the electron beam current is increased to obtain a throughput competent for mass production.
To mitigate the limitation on throughput, many years ago it was proposed to use a plurality of electron beams each having a small current to expose patterns onto a wafer in parallel, instead of using one electron beam with a large current, such as in U.S. Pat. No. 3,715,580. For structuring a multi-beam apparatus using a plurality of electron beams, one critical problem is how to separately focus multiple electron beams individually and in parallel. Configuring multiple single-beam columns (MSCs) into one multi-beam apparatus is one solution naturally thought out at first. Because the spatial interval between every two adjacent beams must be large enough to physically accommodate two single-beam columns in parallel, the number of electron beams available for a wafer or a mask is not sufficient for mass production. As an alternate to use the MSCs, configuring a multi-axis lens to individually focus multiple electron beams in parallel is a promising way for using more electron beams. A multi-axis lens can reduce the beam interval by 50%, thereby almost doubling the apparatus throughput. Consequently, a multi-beam apparatus using a multi-axis lens can obtain a much higher throughput without degrading spatial resolution in comparison with using MSCs.
In U.S. Pat. No. 3,715,580, Maekawa et al. propose a multi-axis magnetic lens for throughput improvement of an IC pattern exposure system. FIG. 1A and FIG. 1B schematically show one embodiment of the multi-axis magnetic lens by a sectional view and a top view. In FIG. 1A, the multi-axis lens 100 comprises a common excitation coil 44, one yoke 43 and a pair of parallel magnetic conductor plates 41 and 42 with a plurality of through round holes in pairs therein. Each paired through round holes respectively in plates 41 and 42 are aligned with each other and have a common central axis, for example the paired holes H1 and h1 and the common central axis A1 thereof. When an electric current is exerted into the coil 44, a magnetic round-lens field will be generated between each paired holes. By this means, many magnetic sub-lenses can be formed in one multi-axis magnetic lens, such as S1, S2 and S3. For each sub-lens, the portions of plates 41 and 42, respectively forming the paired holes, are two pole-pieces of the sub-lens. Each magnetic sub-lens has an optical axis coincident with the common central axis of paired holes thereof and can focus an electron beam entering the sub-lens along the optical axis, such as beam B3 along the optical axis A3 of the sub-lens S3.
In the foregoing multi-axis magnetic lens, the magnetic flux leakage between each paired holes depends on the positions thereof on plates 41 and 42, geometrical shapes and magnetic permeability of plates 41 and 42, and the distribution of all the holes on plates 41 and 42. Hence, the magnetic fields of all the sub-lenses are fundamentally not a pure round-lens field and different from each other in distribution pattern and strength, no matter the shapes of plates 41 and 42 are circular as shown in FIG. 1B or not. Consequently, there are two inherent issues which hinder all the electron beams to obtain superior resolutions similar to that of a single beam focused by a single-axis lens which is fundamentally axisymmetric.
The first issue is a non-axisymmetry of the magnetic field in each sub-lens. The magnetic field distribution of each sub-lens degenerates from axial symmetry to a rotation symmetry and/or n-fold symmetry. In terms of Fourier analysis, the magnetic field comprises not only an axisymmetric component or called as round-lens field, but also a lot of non-axisymmetric transverse field components or called as high order harmonics, such as dipole field and quadrupole field. Only the round-lens field is necessary for focusing an electron beam, and the other components are undesired due to their impairment on beam focusing. The dipole field deflects the charged particle beam, thereby making the beam land on the image plane with position error, additional tilt angle and deflection aberrations. The quadrupole field adds astigmatism to the beam focusing. To compensate the influence of each high order harmonic, at least one additional element generating the same type field is required for each electron beam.
The second issue is the focusing power differences among all the sub-lenses if all the through round holes are same in geometry. The round-lens fields of all the sub-lenses are not equal to each other due to the differences in magnetic flux leakage. The sub-lens closer to the geometrical center of the plates 41 and 42 has a weaker round-lens field. For instance, compared with the sub-lenses S1 and S3, the sub-lens S2 has a weaker round-lens field. Due to the round-lens field difference, the beams B1, B2 and B3 respectively passing through the sub-lens S1, S2 and S3 are not focused onto a same image plane.
Many scientists propose methods to fundamentally mitigate or even eliminate the two issues per se. In U.S. Pat. No. 6,750,455, Lo et al. use a plurality of dummy holes to improve the local structure symmetry of each sub-lens. However this method makes the multi-axis magnetic lens system bulky. In U.S. Pat. No. 8,003,953, Chen et al. form a permeability-discontinuity (simply expressed as PD hereafter) unit inside each hole of every sub-lens to eliminate non-axisymmetric transverse field components inside every sub-lens and the focusing power differences among all the sub-lenses. For the sake of clarity, the foregoing unit is named as the first-type PD unit hereafter. A first-type PD unit comprises at least one non-magnetic annular layer and at least one magnetic annular layer all in alternate arrangement, and one of the magnetic annular layers is innermost. Inside a first-type PD unit, a magnetic annular layer is immediately enclosed by a non-magnetic annular layer and/or immediately encloses a non-magnetic annular layer so that permeability spatially alternates between 1 and much higher than 1 at least one time from the outermost layer to the innermost layer. Inside every hole where a first-type PD unit is formed, the outermost layer thereof adjoins the inner sidewall of the hole, and the innermost magnetic annular layer becomes a pole-piece instead of the portion forming the hole. The spatial alternation of permeability between 1 and much higher than 1 increases axial symmetry of the scale potential distribution inside the inner hole of the innermost magnetic annular layer and consequently reduces non-axisymmetric transverse field components of the sub-lens field.
FIG. 2A takes the sub-lens S3 as an example to show a simple embodiment of first-type PD formed inside the hole H3 in the upper magnetic conductor plate 41. One magnetic ring R3 with high permeability is inserted into the hole H3 with a non-magnetic radial gap G3 so as to form a first-type PD therein. The gap G3 is either a vacuum space, or filled with a non-magnetic material. Inside the first-type PD unit, permeability increases from 1 to permeability uR3 of the magnetic ring R3. The magnetic ring R3 is preferred to fully cover the sidewall of the hole H3, as shown in FIG. 2B. FIG. 2C further shows two first-type PD units respectively inside the paired holes H3 and h3 of the sub-lens S3. The two magnetic rings R3 and r3 of the two units become two pole-pieces and are aligned with each other to have a common central axis Z3. A magnetic field along the axis Z3 is generated through the non-magnetic gap between these two pole-pieces R3 and r3, and the axis Z3 becomes the optical axis of the sub-lens S3. One of two pole-pieces can be extended into the inner hole of the other pole-pieces so as to eliminate the non-axisymmetric transverse field components in the axial gap between two pole-pieces. The thicknesses of gaps G3 and g3 are small enough to keep a sufficient magnetic coupling for making the round-lens field strong enough, and large enough to minimize non-axisymmetric transverse field components to a negligible level inside the inner holes O3 and o3 of the two pole-pieces R3 and r3 respectively. In such a way, the non-axisymmetric transverse field components in the areas inside each sub-lens are reduced to a level much lower than that in FIG. 1A. The round-lens field differences or called as focusing power differences among all the sub-lenses are eliminated by appropriately choosing thickness differences of non-magnetic layers of first-type PD units.
Accordingly, Chen et al. employ a plurality of first-type units to form a multi-axis magnetic immersion objective in two patent applications list in the cross-reference, which comprises a plurality of immersion objective sub-lenses so that a plurality of charged particle beams can be individually and in parallel focused onto a specimen surface with small aberrations. Next, in the provisional application list in the cross-reference, Ren et al. simplify a foregoing multi-axis magnetic immersion objective in structure and generalize a first-type PD unit in material and constitution. Each sub-lens of the simplified multi-axis magnetic immersion objective has only one first-type unit rather than two. For generalizing a first-type PD unit in material, all or some of the non-magnetic layers can respectively be replaced by a weakly-magnetic layer, and thereby forming a second-type PD unit or a hybrid-type PD unit. For generalizing a first-type PD unit in constitution, one or more layers can have sub-layers. Each of the foregoing ways can make a multi-axis magnetic lens easier and more flexible in manufacturing.